A vessel moves in six axes, three translational (surge, sway and heave) and three rotational (roll, pitch and yaw). These six axes are shown in FIG. 1. A DP system for a surface vessel usually controls only the three movements in the horizontal plane, namely surge, sway and yaw, but it may need to take into account measurements on all six axes.
The fundamental components of a DP system are: one or more position reference systems to measure the vessel position and heading; thrusters to apply control action; and a controller to determine the required thrusts. The object of a DP system is not to hold the vessel absolutely stationary, but to maintain its station within acceptable limits. The magnitude of the permitted position variation is dependent upon the application and on operational concerns. In many applications a loss of position beyond the acceptable limits may have a severe impact either on the safety of personnel or equipment, or on the environment. It is vital, therefore, that adequate measures are taken to maintain the integrity of the DP system as far as is reasonably possible.
Safe operation in DP relies upon measurement of the vessel position and heading at all times. In order to ensure that this is true, even under fault conditions, all measurement systems include redundancy. Physical redundancy requires the replication of equipment to ensure that a single failure of any piece of equipment will not result in complete failure of the overall system and allows faulty equipment to be by-passed using the redundant hardware. The parallel redundant systems must be independent—i.e. no single failure mode should be capable of disabling the overall system.
The DP system combines all available measurements of position, from whatever source, into a single estimate of vessel position. The algorithm for combining the measurements can be based on a Kalman filter.
The sources of measurements can include a wide variety of position measurement equipment (PME) such as gyrocompasses (which offer compact, reliable and accurate measurement of vessel heading (yaw), independent of outside disturbances), taut wires, satellite navigation systems (which include global positioning systems (GPS) and differential GPS (DGPS)), inertial navigation systems (INS), and hydro-acoustic positioning systems.
An INS uses measurements of acceleration to estimate the motion of a vessel in an inertial reference frame. However, due to physical processes associated with the acceleration due to gravity, plus inherent accuracy and noise within the devices themselves, a degree of drift on position measurements will always be present. This means that there is a need for periodic updates to the INS estimates of drift. These updates can be supplied by another PME unit such as a hydro-acoustic positioning system or a satellite navigation system, for example.
INS and Hydro-Acoustic Positioning Systems
The problems of deep-water acoustics are well known (Stephens, R. I. “Aspects of industrial dynamic positioning: reality-tolerant control”, IFAC Conference on Control Applications in Marine Systems, CAMS 2004, 7-9 Jul. 2004, Ancona, Italy , pp. 41-51). The depth introduces long ping cycle times due to the distance for the sound to travel, unless so-called ping stacking is employed. Deep water also increases the cost of replacing batteries in transceivers, which starts to become a significant proportion of the overall cost of the hydro-acoustic positioning system. By increasing ping cycle times the battery life can be extended. Ping stacking only serves to reduce the battery life. Unfortunately, the increased ping times can adversely affect the DP control. This is true even though it is straightforward to configure the DP Kalman filter controller to make use of long measurement update rates. It tends to lead to higher thrust usage as the DP system detects deviations later and has to apply greater adjustments in thrust. Any changes in environmental forces or small inconsistencies in the vessel model of the DP system are exacerbated by a long ping time. There is also the possibility that aliasing effects due to long position measurement periods will introduce increased noise into the position measurements.
Integrating an INS into the hydro-acoustic positioning system can allow the long ping times to be reduced by using the INS to fill-in between pings.
In order to investigate the benefits of this approach, a number of tests have been conducted on a two small vessels utilizing a hydro-acoustic positioning system, combined with a PHINS inertial sensor (both items being supplied by IXSEA of 55, Avenue Auguste Renoir, 78160 Marly le Roi, France). The tests were performed in ultra-short baseline (USBL) mode in 15 m water depth off Brest (France) and in 1000 m water depth off La Ciotat (France). In the deeper water, the shortest ping cycle time, without ping stacking, was 3 s; therefore data was collected at that rate. Both raw acoustic data and PHINS corrected positions were logged. An additional signal was generated from the PHINS using acoustic measurements sampled every 21 s rather than 3 s.
While it was not possible to install a DP system on the vessel in the time-scale of the trials, the results have been post-processed using a simulation of the DP system to estimate the behaviour of a vessel controlled by a DP system under the same conditions. The measured errors from the INS trials have been imposed on the simulation and comparisons made between raw acoustic signals at 21 s updates and acoustics plus INS.
FIG. 2 shows a comparison of the vessel position errors during the simulation runs. It shows that the position keeping is significantly improved by using the INS corrections. The standard deviation of X-axis errors for acoustics only is 6.8 m while using INS reduces this to 3.4 m.
An even greater improvement is achieved in the thrust demands from the DP system for the same scenario. FIG. 3 compares the thrust demands with and without the INS corrections. It shows that thruster usage is dramatically reduced when the INS is filling-in between pings. This reduction in the thrust demand variation means less mechanical fatigue, less wear and reduced maintenance. Another consequence is a reduction of the thruster-generated noise in the water, which means better acoustic detection through an improved signal to noise ratio, not only providing more accurate and secure acoustic positioning, but also enabling operations in deeper waters.
A further illustration is gained by comparing the fuel consumption during DP. By estimating thruster power P from thrust T using the approximate relationship: P∝T1.5, the relative fuel consumption can be estimated. FIG. 4 shows the evolution of relative fuel consumption calculated in this way. It reveals that the INS can reduce fuel consumption by a factor of five or more.
INS and Satellite Navigation Systems
The use of INS with satellite navigation systems (both GPS and DGPS) provides the following benefits: detection of GPS failures, removal of erroneous ‘jumps’, ride-through for temporary outages, and reduced thrust demand implying lower fuel consumption in nominal operation.
A common occurrence using GPS and DGPS is a jump in the position estimate. This can occur when the visible satellite constellation changes, either as the result of satellites rising or setting, or due to shielding from nearby objects. Typical examples of the latter include passing under a bridge or approaching a platform. These jumps are often negligible, but sometimes become significant. For example, FIG. 5 shows a short jump of about 3 m and a short outage of about 15 s, which occurred in open water in the North Sea. The severity of a jump depends upon the operational situation of the vessel. Under most conditions a jump of 3 m is not problematic. During a close approach to a platform or other vessel, however, even small jumps can be ‘uncomfortable’. The DP system includes algorithms for error detection, including detection of noise, jumps and drift. Though these algorithms are sophisticated in their own way, the most reliable forms of error detection rely on comparison of two, three or more PME. The INS, as it is not based on a model but on real acceleration measurements, not only acts as a filter on the DGPS measurements, but also rejects data during short term jumps, and fills-in for short outages.
Use of a high-quality INS in conjunction with a DGPS receiver reduces the level of high-frequency noise on the measured position. This has the effect of reducing the noise on the thruster demands, in the same way as the INS reduced the noise of the acoustics discussed above. In the case of the DGPS receiver, the effect is less dramatic since the noise is initially smaller. This reduction of noise is not the same as filtering: filtering introduces extra phase-lag into the control system whereas the INS is enhancing the position accuracy of the measurements without introducing lag.
Sea trials have been conducted on a 7000 t vessel utilizing a DP system supplied by Converteam UK Ltd of Boughton Road, Rugby, Warwickshire CV21 1BU, United Kingdom and a DGPS receiver combined with a PHINS inertial sensor. For part of the trial, the vessel was held in a constant position under full control of the DP system with the DGPS receiver as the only PME, followed by a period with the combination of the DGPS receiver and the PHINS inertial sensor as the only PME. FIG. 6 shows the thruster demands for the X and Y axes during the two periods of operation. There is noticeably less noise for the combination of the DGPS receiver and the INS.
In order to compare the expected fuel consumption with and without the INS corrections, the thrust demands were used to estimate a relative fuel consumption using the relationship P∝T1.5 as before. The results of the estimation, for the 10 min periods of the trial are shown in FIG. 7. The system without the INS uses 40% more fuel than the combination of the DGPS receiver and the INS.
The usefulness of an INS during an outage of other PME depends on its drift. This drift is a function of inertial measurement unit (IMU) quality, calibration and correction. The short term accuracy of an INS derives from the accuracy of its accelerometers, while the longer term accuracy derives from the gyro accuracy. The position is defined by the double integration of the accelerometers, so the position drifts according to the square of time and the stability of the accelerometers.
Previously, outage data has been obtained for a stationary INS unit (Paturel, Y. “PHINS, an all-in-one sensor for DP applications”, MTS Dynamic Positioning Conference, 28-30 Sep. 2004, Houston, United States of America). However, it will be readily appreciated that on a sea-borne vessel the INS will never be stationary. A series of tests were therefore carried out using a GPS receiver and an INS in constant oscillatory motion, simulating bad weather. During the tests, the GPS input to the INS was removed at periodic intervals and the positions of the INS and GPS compared over a period of ‘outage’. Typical results from these tests are shown in FIGS. 8 and 9. FIG. 8 shows the evolution of the INS drift with time during outages of 120 s and 300 s. The results compare well with previous investigations of stationary systems.
FIG. 9 shows the distribution of the errors after 120 s and after 300 s. The distribution of drift errors exhibit the shape of the Rayleigh distribution, which is characteristic of processes formed from the sum of squares of Gaussian distributed sources—because the drift distance is the sum of squares of the deviations in North and East directions.
To get an idea of the relative drifts of a vessel with no PME and one using only INS, it is possible to estimate the force required to move a vessel off-station by the same amount as the observed INS drift. Taking the worst case from FIG. 8, distance traveled s=22 m after time t=300 s, the equivalent constant acceleration a can be calculated from a=2 s/t2=4.9×10−4 m/s2. For a typical supply vessel of displacement ∇=4000 t, the force F required to achieve this acceleration would have been F=∇a=2.0 kN. This is less than about 1% of the likely onboard thrust, suggesting that under moderate conditions, a vessel with no PME is likely to drift far more quickly than the INS. In addition, the drift of the INS is based on real physical measurements of the accelerations, not on a model which would become degraded in case of non-nominal conditions like bad weather with large waves, or breaking of cables or an umbilical that would be linked to the platform.
It should be noted that the intervals between the trials presented in FIG. 8—i.e. periods during which the GPS was available again—were between 30 s and 300 s with no obvious difference between the two. This suggests that the self-alignment of the PHINS inertial sensor is excellent, and the interval between outages is unlikely to be a problem in practical situations.
Known DP System Architecture
Due to its dependence on position measurements continually to estimate the errors in the accelerometers, it is not possible to treat an INS alone as an independent PME unit. It will always be dependent on one or more of the other PME units. So, to keep independence between the PME units, the general practice is that an INS unit should be tightly coupled with a single PME unit, for example a hydro-acoustic positioning system.
FIG. 10 shows an example of a typical architecture for a DP system. The DP system receives data from a plurality of PME units—in this case from two satellite navigation systems labelled GPS1 and GPS2, a hydro-acoustic positioning system labelled Acoustics1—and an INS unit. The data supplied by the PME units and the INS unit will normally represent position measurements, but it may also represent acceleration measurements or velocity measurements, for example. Additional information such as status indicators, data quality indicators and statistical information may also be included in the data that is supplied by the PME units and the INS unit.
The INS unit receives data from a second hydro-acoustic positioning system labelled Acoustics2 and the DP system treats the combination of the second hydro-acoustic positioning system and the INS unit as a single input (i.e. as a single PME unit). The data from the second hydro-acoustic positioning system is used to correct the drift in the INS unit.
In this configuration it is important for information to be passed to the DP system concerning the quality of the combination of the second hydro-acoustic positioning system and the INS unit. For example, the DP should be warned if the INS unit loses the data from the second hydro-acoustic positioning system.
It will be readily appreciated that one of the two satellite navigation systems GPS1 and GPS2 can also be combined with an INS in the same way. In other words, the INS unit could receive data from a GPS receiver to correct the drift in the INS unit.
An alternative architecture is shown in FIG. 11. In this architecture the DP system receives data from a plurality of PME units—in this case from two satellite navigation systems labelled GPS1 and GPS2, two hydro-acoustic positioning systems labelled Acoustics1 and Acoustics2—and an INS unit. The INS unit can receive data from all of the PME units. However, it is important that the INS unit uses the data supplied by only one of the PME units at any one time to maintain its independence from the others. The alternative architecture shown in FIG. 11 offers the main advantage of flexibility to the operator. The operator has the ability to choose between two possible configurations, depending on his application. For instance, if acoustics is required the operator can choose a configuration where the INS unit receives data from one of the hydro-acoustic positioning systems. Otherwise, the operator may choose a configuration where the INS unit receives data from one of the satellite navigation systems. The data from the PME unit will be used to correct the drift of the INS unit.
The status indicators sent to the DP system must include enough information for the DP system to determine the configuration of the PME units and the INS unit. In particular, the DP system must be able to ensure that the particular PME unit used in the INS calculations is not used by the Kalman filter to derive the vessel position estimates. For example, if the INS unit is configured to receive position measurements from the second hydro-acoustic positioning system (Acoustics2) then the DP system will not use any position measurements supplied directly from the second hydro-acoustic positioning system to derive the vessel position estimates. The DP system may, however, use any additional information that is supplied directly from the second hydro-acoustic positioning system. Any position measurements supplied by the INS unit will, of course, be used by the DP system to derive the vessel position estimates.
The alternative architecture of FIG. 11 is already feasible with existing equipment because the PHINS inertial sensor includes multiple input ports.